We derisively brought up modal logic, "possible worlds" talk, on our Frege episode, and we'll likely do an episode on that if we're still podcasting a couple of years down the road, but if you want to know a bit more now, you could look at Wikipedia here, or better yet, the Stanford Encyclopedia of Philosophy. Really, though, why not take the easy route and learn about David Lewis from the Philosophy Bro?
Think of all the ways things could have been. The mundane ways, sure: you could have preferred tea instead of coffee, that lightbulb could have been blue instead of yellow, Steve Jobs could have founded Windows. Whatever. But shit could have been way crazier than that. We could have tentacles. Gravity could have been twice as strong. Geometry could be parabolic. Electrons could be replaced with - who the fuck knows, really.
Are you ready for the fun part? Because here comes the fun part: there are uncountably infinitely many worlds, and there's a world for every single fucking possibility. Remember how Hume said that nothing was ever necessarily connected to anything else? Yeah. As long as something doesn't contain any blatant contradictions, there is a world in which it's reality.
Peace out, bro.